a study of nanosized zinc oxide and its nanofluid

PRAMANA c© Indian Academy of Sciences Vol. 78, No. 5— journal of May 2012

physics pp. 759–766

A study of nanosized zinc oxide and its nanofluid

D K SINGH1, D K PANDEY2,∗, R R YADAV3 and DEVRAJ SINGH4

1Department of Physics, Government Inter College, Bangra, Jalaun 285 121, India2Department of Physics, P.P.N. College (P.G.), Kanpur 208 001, India3Department of Physics, University of Allahabad, Allahabad 211 002, India4Department of Applied Sciences, Amity School of Engineering and Technology, Bijwasan,New Delhi 110 061, India∗Corresponding author. E-mail: [email protected]

MS received 19 April 2011; revised 20 October 2011; accepted 11 January 2012

Abstract. The synthesis and characterization of nanosized zinc oxide and its nanofluid in apolyvinyl alcohol (PVA) matrix have been done in the present investigation. Crystalline zinc oxidenanoparticles are synthesized using single-step chemical method while the nanofluids are preparedby the dispersion of nanoparticles in PVA solution using an ultrasonicator. The prepared nanopar-ticles are characterized using X-ray diffraction, SEM–EDX and UV–visible spectrum. The particlesize distribution measurement is carried out by acoustic particle sizer. The ultrasonic velocitiesare measured in the synthesized nanofluid under different physical conditions using an ultrasonicinterferometer. It is found that the degree of crystallinity of nanoparticles depends on the evapora-tion rate during its synthesis and ultrasonic velocity has non-linear relation with temperature for thepresent nanofluid.

Keywords. ZnO nanoparticles; ZnO–PVA nanofluid; structural, optical and ultrasonic properties.

PACS Nos 42.25.D; 43.35.+d; 61.05.cp

1. Introduction

Zinc oxide is a wide band-gap semiconductor. Several studies have been done for its char-acterization and application [1–4]. Metal oxide nanostructures can be used in solar cells,electroluminescent devices, electrochromic windows and chemical sensors [5]. The ZnOnanostructures can be found in several forms like rod/wire [6,7], thin film [8] and parti-cles [9–11]. The nanostructured ZnO possess high surface area as well as good electrical,electrochemical and structural properties. The synthesis of ZnO nanoparticles and theirstructural/optical characterization can be seen in literature [9–11]. The ZnO powders atnanoparticle size can be used as antibacterial agents [12].

The sound propagation through random media (colloidal suspensions, porous mate-rials and nanofluids) has also been a subject of great interest recently. Nanofluid is a

DOI: 10.1007/s12043-012-0275-8; ePublication: 25 April 2012 759

D K Singh et al

colloidal suspension of nanoparticles in a polymer matrix. Nanofluids have been of greatinterest due to their broad applications in different fields [13]. The preparation and char-acterization of ZnO nanoparticles, ZnO–ethylene glycol and ZnO–water nanofluids arereported in [13–15]. The anomalous behaviour of ultrasonic velocity in sintered ZnOprovides information about the pore size, pore shape and their distribution [16]. Veryfew nanofluids and magnetic fluids are found to be characterized using ultrasonic tech-nique [17–19] and no work has been reported for determining the ultrasonic propertiesin ZnO nanofluid to the best of the author’s knowledge. In the present investigation, wehave chosen ZnO–PVA nanofluid for the characterization. Initially, ZnO nanoparticlesare synthesized through chemical route and characterized by XRD, SEM and UV–visibleabsorption spectrum. Later on, ZnO nanofluid is synthesized using polymer solution ofPVA with the help of ultrasonicator. We have measured the particle size distribution in theprepared nanofluid for the justification of particle size. The ultrasonic velocity measure-ments are performed for ultrasonic characterization of the prepared fluid. The obtainedresults are analysed for the characterization.

2. Sample preparation and measurements

Zinc oxide nanoparticles were prepared successfully by the one-step solvothermal reac-tion of zinc acetate in alcohols. Zinc acetate dihydrate (13.4 mmol) was dissolved inmethanol and a solution of potassium hydroxide was prepared by dissolving potassiumhydroxide (28.96 mmol) in methanol. The potassium hydroxide solution was added drop-wise to the zinc acetate solution at 52◦C under vigorous stirring. Nanoparticles started toprecipitate and the solution became turbid. The heater and stirrer were removed after 2 hand the solution was allowed to sit for another 2 h. The ZnO nanoparticles were settled atthe bottom and the excess mother liquor was removed. The precipitate was washed twicewith methanol (50 ml) and collected by centrifugation process and allowed to dry at roomtemperature for one day. The obtained solid was crushed to get a white powder and theobtained powder was dispersed in PVA with ultrasonicator. The dispersed solution wastranslucent and stable for up to 2 weeks. Aggregation was found to occur at this stage.The size and shape of the particles were controlled by evaporation rate. Two samples ofZnO nanoparticles were prepared by controlling the evaporation rate.

The X-ray diffraction patterns of the powdered samples of the ZnO nanoparticles wereanalysed using an X’Pert-Pro, PANalytical (with λ=1.5405 Å Cu-Kα radiation) operat-ing at room temperature. A scanning electron microscope of Oxford model Leo 1550was used for the morphological and average particle size study. The UV–visible absorp-tion spectrum was recorded using Lambda 35, Perkin–Elmer double beam UV–visibleabsorption spectrometer for optical characterization.

1, 2 and 5 wt% of the ZnO nanoparticles (prepared at low evaporation rate) weredispersed in a solution of 3 wt% PVA using ultrasonicator to form the nanofluid. Theparticle size distribution measurements were done by an acoustic particle sizer modelAPS-100 (Matec Applied Sciences, Massachusetts, USA). Ultrasonic velocity was mea-sured at 2 MHz using ultrasonic interferometer in the temperature range of 25–65◦C.Water was circulated around the sample using a specific thermostat. The measured value

760 Pramana – J. Phys., Vol. 78, No. 5, May 2012


of ultrasonic velocity was accurate to ±0.1% with an error of measurement of ±0.5◦C intemperature.

3. Results and discussion

3.1 Structural and optical characterization

The structural analyses of the ZnO nanoparticle samples, synthesized at low andhigh evaporation rates were carried out by XRD and UV–visible absorption spectrummeasurements. The XRD patterns of both the samples are shown in figure 1.

X-ray diffraction pattern (figure 1) shows that narrower and higher intense peaks areobtained in the diffraction pattern for samples formed at low evaporation rate than forsamples formed at high evaporation rate. Thus the degree of crystallinity and particlesize of the samples increase with decrease in evaporation rate. The crystallite/particlesize has been calculated using the Debye–Scherrer equation for all diffraction peaks. Theparticle size of the sample is found ranging between 35 and 50 nm. The intensity peaksof XRD pattern are coordinated with JCPDS file number 790206 and it is confirmed thatthe crystalline sample is of ZnO which is hexagonal in structure with lattice parametersa = 3.249 Å and c = 5.206 Å. The size and morphology of the ZnO nanoparticles havebeen determined using scanning electron microscopy. Figure 2 shows the typical scanningelectron micrograph (SEM) images of ZnO nanoparticles. The SEM image shows randomdistribution of the ZnO nanoparticles having non-spherical shape and diameter in therange of 30–60 nm. The EDX spectrum (figure 3) of the ZnO nanoparticles reveals thatour sample contains zinc, oxygen, gold, and potassium as constituent components. Thetrace of gold is due to the sample coating during SEM imaging and potassium can broadly

Figure 1. XRD patterns of the powder sample of ZnO nanoparticle at high and lowevaporation rates.

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Figure 2. SEM image of the agglomerated ZnO nanoparticles.

be attributed to the fact that potassium hydroxide was used in the reaction process. Thehigh-intensity peaks for zinc and oxygen justify that the sample contains mainly ZnO.

The room-temperature UV–visible absorption spectrum of the ZnO nanoparticles isrecorded in the wavelength range of 250–800 nm. Figure 4 represents the UV–visibleabsorption spectrum of the ZnO nanoparticles. The spectrum has a peak at 367 nm(3.38 eV). The absorption peak for 40 nm ZnO nanoparticles has been reported at 361 nm(3.44 eV) elsewhere [20,21]. The absorption peak for 5–6 nm ZnO nanoparticles has beenfound to be between 360 and 365 nm [5,9]. The variation in absorption peak is due to vari-ation in particle size. The band gap for normal-sized ZnO is 3.3 eV. Thus at nanoscale,the band gap increases for the ZnO.

Figure 3. EDX spectrum of ZnO nanoparticles using zinc acetate as the precursormaterial.



Figure 4. UV–visible absorption spectrum of the ZnO nanoparticles.

3.2 Particle size distribution and ultrasonic velocity

The particle size distribution measured using acoustic particle sizer (APS-100) is plottedin figure 5, which clearly explains that the size of the suspended particle in the PVAsuspension (nanofluid) is in the range 35–50 nm. The maximum numbers of particles arein approximately 42 nm size. This confirms that the prepared solution is a nanofluid. Itis observed that the present nanofluid is found to be stable for more than one week. The

Figure 5. Particle size distribution curve.


D K Singh et al

stability of the nanofluid suspension can be accurately determined with Zeta potentialmeasurement [22].

The ultrasonic velocities measured for pure PVA solution and the three nanofluids inthe temperature range 25–65◦C are shown in figure 6. The velocity curves indicate that theultrasonic velocity in the samples increases with the temperature and it becomes approx-imately constant after 55◦C. Ultrasonic velocity is found to be higher for the nanofluidsthan for pure PVA solution. It is also observed that the magnitude of velocity increaseswith the particle volume fraction.

The behaviour of ultrasonic velocity can be understood with its related quantities. Theultrasonic velocity is well related to bulk modulus (B), compressibility (k), and density(ρ) of the medium (V = √

B/ρ = 1/√

k ρ; k = B−1 = (λ + 2μ)−1; λ and μ: Lamemoduli). The density, compressibility and Lame moduli of a fluid medium are alteredby the dispersion of nanoparticles and are functions of particle volume fraction [23–25].Hence, the enhancement in velocity by the dispersion of nanoparticles is due to change indensity, compressibility, bulk modulus and Lame moduli of the nanoparticle suspensionwith volume fraction. The velocities of the present nanofluids are found greater than thevelocity in pure matrix state, which indicates that by the dispersion of solid particles –there is a change in density and bulk modulus. An increase in bulk modulus or decreasein compressibility is attributed to the fact that strong cohesive interaction forces act amongthe molecules and atoms after the dispersion of ZnO nanoparticles in the PVA matrix.

The velocity of the nanofluid is independent of particle size in low-frequency region[24–26]. Here all the nanofluids are prepared with ZnO nanoparticles primed at low evap-oration rate and temperature-dependent velocity is measured at low frequency (2 MHz).Thus it may be concluded that temperature-dependent velocity at low frequency in ZnO

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20 30 40 50 60T(0C )

Ultr

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ic V

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2.0wt% ZnO

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Figure 6. Velocity vs. temperature for pure PVA solution and nanofluids.



nanofluid depends only on the particle volume fraction. The ultrasonic velocity in thenanofluid is a quadratic function of temperature at low frequency [25].

V = V0 + V1T − V2T 2, (1)

where V0 is the ultrasonic velocity at initial temperature, V1 and V2 are the absolutetemperature coefficients of velocity and T is the temperature difference between exper-imental and initial temperature. The first two terms in eq. (1) are found normally forsimple liquid system, but the third non-linear term is caused by nonlinear change in bulkmodulus/density of nanofluid system with temperature. It affirms that eq. (1) is alsojustified for the present nanofluid.

4. Conclusions

The following conclusions can be drawn on the basis of the above discussion:

• The XRD pattern shows that the degree of crystallinity of the ZnO nanoparticledepends on the evaporation rate during its synthesis.

• The band gap of ZnO increases at nanoscale.• The temperature variation of ultrasonic velocity in ZnO–PVA nanofluid mainly

depends on the particle volume fraction of the dispersed particles in low-frequencyregion and is larger than the velocity for pure matrix. A non-linear relation betweenvelocity and temperature is found for the present nanofluid samples.

The present results along with known physical properties of the nanocrystalline ZnO,offer a new dimension for further study and characterization of nanofluids.

Acknowledgements

The authors are thankful to the University Grants Commission-India (Project No. 32–18/2006 (SR)) for the financial support. The authors owe special thanks to Mrs NeeraBhutani, ASET, New Delhi for reading the manuscript meticulously.

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a study of nanosized zinc oxide and its nanofluid

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